[app-shells/elvish] doesn't register itself in /etc/shells

[nelsam]

After messing around with elvish for a few days, I decided to set it as my default shell for a while, see if I want it to become my main shell for a while. chsh was definitely not okay with that, though - and I found out it's because the ebuild doesn't register elvish in /etc/shells.

I put together a fix here, copy/pasting the pkg_postinst from another shell. If I could get repoman running locally, I'd have submitted a PR, but 🤷‍♂️